This invention relates to an improved process for beneficiating phosphate rock. More particularly, this invention relates to an improved method for concentrating (extracting) phosphate minerals from their ores in the flotation process.
One source of the phosphate mineral is a substance known as phosphate rock. Phosphate rock is found in significant concentrations in mid-Florida area sedimentary deposits. Within the phosphate mining industry the rock is generally surface mined by the use of draglines, dredges or other apparatus.
The phosphate matrix, also known as phosphate ore, that is mined is slurried with high pressure water and pumped to a processing plant where phosphate is extracted. The processing plant separates clays, silica and other diluents from the phosphate rock in order to produce various sizes and grades of salable phosphate rock product. Typically an operation would produce a pebble product, an intermediate product and a flotation concentrate. The size distributions and grades of these products are to a degree phosphate ore and processing facility specific with the following descriptions representing a typical operation. When the phosphate ore slurry reaches the plant it is treated in a washing step where particles larger than approximately 14 Tyler mesh are separated from the remainder of the phosphate ore. This size fraction is typically known as phosphate pebble and has a range of bone phosphate of lime (BPL) values of about 50 to 70.
The material smaller than 14 Tyler mesh is advanced to further sizing processes where clays and other minerals smaller than approximately 200 Tyler mesh are removed and transferred to waste slimes settling areas. Particles larger than 20 Tyler mesh are retained as an intermediate product which typically ranges from 40 to 65 BPL. The methods used to achieve the separation of this intermediate product vary widely and include assorted hydrosizers, spiral classifiers, screens, other mineral sizing techniques and various combinations of these techniques as required by ore characteristics and individual site preference.
The remaining size fraction represents a fairly low grade material which is not usually salable without additional concentration/upgrading. The fraction that falls between 20 and 200 Tyler mesh represents a substantial percentage of the phosphatic values in the ore. A typical range of about 5 to 40 BPL can be attributed to this size fraction depending on geography and specific matrix characteristics.
This size fraction is typically referred to as flotation feed and is usually concentrated by froth flotation. Typically this is accomplished by an initial rougher flotation step where all recoverable phosphate and a fairly high quantity of silica and other diluents are floated, followed by one or more cleaner flotation step(s) to remove the silica and other diluents from the phosphate concentrate. These flotation step(s) produce a phosphate concentrate product with enhanced grade and salability. Typical rougher flotation concentrates range from 50 to 65 BPL with cleaner flotation concentrates ranging from 65 to 75 BPL.
In the flotation process the flotation feed might be subjected to further sizing operations prior to rougher flotation. These sizing operations vary widely in scope and intent with the flotation process actually beginning when the flotation feed is forwarded to conditioning vessels where reagents are added and the mixture agitated.
In the conditioning step flotation feed is brought into contact with various organic substances including fuel oils, tall oil fatty acids and combinations thereof. These reagents are added in an environment with a controlled pH, which allows the organic components to preferentially adhere to the phosphate containing components, while not preferentially coating silica and other such components. The objective here is to make the phosphate mineral portion hydrophobic so that the phosphate rock component will be floated once the conditioned and reagentized feed enters the flotation cell(s). In the flotation cell(s) the reagentized phosphate is attached to air bubbles in an agitated pulp which then floats off in an overstream froth containing phosphate minerals, some sand and other diluents. Unrecovered phosphate, sand and other minerals are rejected as the unfloated (sink) portion to the rougher tailings stream.
The phosphate which is floated, the rougher concentrate, can be upgraded further to enhance the phosphate concentration. Most operations advance the rougher concentrate to a de-oiling step where an acid, such as sulfuric acid, is added prior to rinsing with clear water to remove the organic substances added in the rougher flotation step. Once de-oiled the rougher concentrate is refloated with an amine and an organic hydrocarbon such as kerosene in a cleaner floation step where silica and other diluents are floated and discarded to the tailings stream as a waste product. The phosphate concentrate is recovered from the sink portion of the flotation cell and transported to storage for eventual sale.
The silica, unrecovered phosphate and other minerals rejected from the flotation process are transported to a tailings waste storage area where these tailings are typically used as fill for land reclamation.
The phosphate extraction/concentration process typically uses large quantities of water which is circulated in a series of settling areas, tailings areas and various holding ponds for reuse prior to the discharge of some water. The quality of this discharged water is of great importance and concern. It should not be in a form that would be harmful to the environment.
In the conditioning step of the rougher flotation process ammonia, sodium hydroxide, ammonium hydroxide or mixtures of these substances are typically used to closely control the pH level in the conditioning vessels.
In U.S. Pat. No. 2,293,640 it is disclosed that a thick aqueous pulp of phosphate ore which consists of about 70% solids is agitated in a solution which consists of fish oil fatty acid, fuel oil and caustic soda. The caustic soda would maintain the pH of this solution in the basic range.
In U.S. Pat. Nos. 4,747,941 and 4,642,181 there is disclosed a method of decreasing the magnesium content of a phosphate ore by means of flotation in conjunction with the use of particular inorganic promoters. The inorganic promoters that are utilized include ammonia and/or sodium carbonate. The beneficiation step that is set forth in these patents is quite different from that which is the subject of the present invention. In the processes disclosed in these patents the ore material is sized and is then mixed with the promoter materials, the fatty acid, fluosilisic acid and a frothing agent. The result is that the phosphate ore particles are made hydrophilic while the fraction of the ore containing the carbonate sand and magnesium impurities is made hydrophobic. Air is introduced into the vessel to form a froth. The froth that is formed produces an overstream of the hydrophobic carbonate, sand and magnesium impurities which are removed from the overstream of the vessel. This is in contrast to the present process wherein the phosphate mineral is made hydrophobic and is removed in the overstream. After the completion of the carbonate float an amine is added as a silica collector, and the slurry is again aerated. Upon being aerated the silica materials float to the surface and are removed in an overstream. The remaining substance in the tank is then primarily phosphate. Interestingly, this patent discloses that sodium carbonate is an inhibitor with regard to the flotation of the phosphate component rather than a promoter of the floation of the phosphate component.
U.S. Pat. No. 4,556,545 discloses a method for conditioning phosphate ores. In this process a hydrocarbon oil, such as fuel oil, is introduced into a pre-stabilization step upstream of the conditioning step. In the pre-stabilization step the hydrocarbon oil is combined with water containing a fatty acid and an alkaline agent under vigorous mixing to produce a stable homogeneous emulsion. The emulsion is then introduced into the conditioning step where it is contacted with phosphate ore. The product of the conditioning step is then conducted to a conventional, downstream floatation step of the operation. The function of the alkaline agent is to saponify the fatty acid. In this regard all alkaline agents are considered to be equivalents. In the preferred embodiment in the examples ammonium hydroxide is used. There is no disclosure with regard to the alkaline agent except to saponify the fatty acid.
Many sources of phosphate ores contain carbonates as one of the contaminants. Many of these are found in the western part of the United States. Thus, in the processing of the phosphate rock material, the carbonate component must be removed. This is accomplished through the use of various flotation techniques. One such technique is disclosed in U.S. Pat. No. 4,486,301. This patent discloses an ore flotation process where the phosphate ore which contains carbonate mineral impurities is subject to froth flotation in the presence of modifying agents such as alkyl phosphoric acids and hydrofluoric acid. The collector substance consists of fatty acids. In this patent, in the separation of the phosphate values from carbonate contaminants, the carbonate is a component of the basic mineral to be removed at an early stage. It is the intent of these processes to separate the carbonate component from the mineral and to thereby leave the phosphate component. These processes do not use sodium carbonate in the rougher stage for the enhanced flotation of the phosphate rock values. Rather, any carbonate that is present is considered to be a contaminant and something that must be removed.
Another operation that is performed on phosphate ores is that of defluorination. Various alkaline materials such as soda ash, sodium sulphate, sodium nitrate, sodium formate, sodium chloride, potassium carbonate, and potassium sulfate have been utilized for the defluorination of phosphate materials. In U.S. Pat. No. 3,058,804 there is disclosed a process for producing defluorinated calcium phosphate. In this process a phosphate rock is mixed with a sodium acid phosphate with the mixture having a moisture content of between about 5% and 15% by weight. These materials are intimately mixed. An alkali metal salt, such as sodium carbonate, is then added. This alkali metal salt is thoroughly mixed with the other components at about 1250.degree. C. to 1300.degree. C. whereby the fluorine content of the calcium phosphate is significantly reduced. In this instance the function of the alkali metal salt, such as sodium carbonte, is to interact with the fluorinated calcium phosphate and to remove most, if not all, of the fluorine content thereof. A related process is disclosed in U.S. Pat. No. 4,152,398 where a defluorinated phosphate is produced that can be used as an animal feed additive.
U.S. Pat. No. 3,078,156 discloses another defluorination process. In this process a phosphoric acid solution is added to a mixture of phosphate rock, Glaubers salt or soda ash and thoroughly mixed. In the following step this mixture is heated to an elevated temperature so as to remove the fluorine content. A further related patent reference with regard to defluorinating phosphate rock is U.S. Pat. No. 3,364,008. In this patent there is disclosed a process whereby a defluorinating agent such as soda ash, or lime is added to phosphoric acid and phosphate rock. The three components are thoroughly mixed and optionally dried and screened. After the optional drying and screening steps, the mixture is fed to a fluid bed reactor. This fluid bed reactor is maintained at a temperature of about 1000.degree. F. to 1300.degree. F. In this reactor fluorine gas is taken off the top of the reactor along with other gases and an agglomerated and defluorinated phosphate product is taken off at the bottom of the reactor.
U.S. Pat. No. 2,839,377 describes another technique with regard to the defluorination of phosphate rock. In this process sodium carbonate is mixed with an aqueous solution of phosphoric acid to produce the treating reagent. The treating reagent is then fed to a rotary kiln along with phosphate rock. During calcination the fluorine content of the phosphate rock is significantly reduced. The result is a calcium phosphate having a very low fluorine content. Another related process is disclosed in East German Patent 200-081-A. In this patent there is disclosed that sodium carbonate and phosphoric acid are mixed with an apatitic phosphate rock. This mixture is then calcined in a rotary tube furnace. The net result is a defluorinated phosphate which can be utilized as an animal fodder additive.
In U.S. Pat. No. 4,609,535 there is disclosed a process for leaching the phosphate values from iron and aluminum phosphates using an alkali metal carbonate solution. The alkali metal carbonate solution contacts the iron and aluminum phosphates and removes the phosphate content. The leachate can then be used to make fertilizers.
U.S. Pat. No. 3,032,189 lists a series of basic inorganic materials for use to adjust the pH of a flotation slurry above 7. These basic organic materials include caustic soda, caustic potash, alkali bicarbonates, such as KHCO.sub.3 and NaHCO.sub.3, and alkali carbonates, such as Na.sub.2 CO.sub.3, K.sub.2 CO.sub.3, and CaCO.sub.3. The patent teaches the use of these materials in a pH range from about 7.5 to about 9.5 and preferably between about 8 and about 9. This patent is directed to the processing of phosphatic ores which contain relatively high concentrations of iron and aluminum, such as those found in Tennessee and West Africa, which cannot be concentrated economically by conventional ore dressing techniques.
None of the patents covering the processing of phosphate rock cited hereinabove are entirely adequate for the processing of phosphate ores from the sedimentary phosphate rock deposits of central Florida. As already noted, some of the processes are undesirable because of their high temperature requirements, some are specifically designed for the removal of magnesium and calcium carbonates, and while others address the beneficiation of phosphate deposits high in specific impurities, such as iron and aluminum.
It is therefore an objective of this invention to provide an improved process for the beneficiation of central Florida phosphate rock.
It is another object of this invention to provide an improved reagent and pH range for the conditioning of flotation feed slurry prior to rougher flotation of phosphate rock.
It is still another object of this invention to provide a flotation system for the processing of central Florida phosphate rock which yields higher phosphate recoveries and generates concentrates with a higher phosphate content than heretofore possible.
It has been found that by using sodium carbonate, the technical grade of which is commercially known as soda ash, as the pH modifier in lieu of those pH modifiers typically used in the prior processes, recovery of phosphate in the flotation process is improved, the grade, that is the phosphate concentration of the rougher flotation product is improved and the quality of the discharge water is improved.